Two and three dimensional modeling of the Wachusett Reservoir, in central Massachusetts, has been conducted for several years at UMass Amherst. CE QUAL W2 is a public domain, two dimensional, laterally averaged hydrodynamic and water quality model. This paper is focused on recent work on predicting the impacts of hypothetical contaminant spills on water quality at the intake that is used to supply a drinking water treatment plant which serves the Boston area using CE QUAL W2. Extensive modeling of the impacts of spill location, time of year, wind, spill density (or temperature) and reservoir operations on the concentration of a conservative tracer constituent was conducted. Results show that spill travel time and concentration at the intake can be significantly affected by these factors. Spills during isothermal, well-mixed, reservoir conditions spread through the reservoir more quickly but at lower concentrations as compared to spills during periods of stratification. Lower density spills at the surface are significantly affected by wind direction and magnitude. Under some conditions, responding to the spill by stopping (or starting) the major Quabbin transfer flow can impact concentrations at the intake. Modeling of reactive constituents shows expected influences of decay based on travel time and surface area.